1. Field of the Invention
The present invention generally relates to the fields of semiconductor fabrication and, more particularly, to high-density plasma (HDP) chemical vapor deposition (CVD) methods and methods of forming semiconductor devices using the same.
2. Description of Related Art
For fabricating semiconductor devices, various processing steps are needed. In particular, chemical vapor deposition (CVD) processes are generally used to fill gaps such as spaces between adjacent raised structures such as transistor gates, conductive patterns, e.g. signal lines, trenches and the like with an insulating material.
As semiconductor device geometries have shrunken over the years, the CVD film's gap-fill capability, i.e., the ability to fill gaps without leaving voids, has become an important factor in the successful production of semiconductor devices. In particular, it has become increasingly difficult to fill narrow, high-aspect ratio gaps or recessed features due to the limitations of existing CVD techniques. This is because most CVD processes tend to deposit more material on the upper region than on the lower region of gap sidewalls and to form undesirable overhangs at the entry of the gap. As a result, the top portion of the high-aspect ratio gaps often closes prematurely leaving voids within the gaps. Such gaps degrade the device characteristics, for example, by trapping undesirable impurities within the gaps.
High-density plasma (“HDP”) chemical vapor deposition (CVD) techniques have been emerged as the technology of choice for filling narrow, high-aspect ratio gaps due to its gap-filling capability. HDP-CVD systems form a plasma having a density that is approximately two orders of magnitude greater than the density of a standard, capacitively-coupled plasma CVD system.
However, the ratio between the heights of the gaps and their widths (the so-called aspect ratio) has continuously increased (e.g. greater than 3). This causes problems such as the formation of voids or reentrant features, e.g., a narrowing at the tops of the gaps. These problems have become a major challenge for successful production of more highly-integrated advanced semiconductor devices.
One approach that the semiconductor industry has developed to improve gap fill capability of the conventional HDP CVD methods is the use of a multi-step deposition and etching process. Such a process is often referred to as a deposition/etch/deposition (dep/etch/dep) process. The dep/etch/dep process divides the deposition of the gap-fill layer into two or more steps separated by a plasma etch step. The intermediate plasma etch step etches the first deposited film more at the upper corners of the film portion more than it does at the sidewalls and lower portions of the gap so that the second deposition step can fill the gap without prematurely closing it off. One such dep/etch/dep process is disclosed in U.S. Pat. No. 6,846,745 (“the '745 invention”). However, the process disclosed in the '745 invention is very complicated and time-consuming, which leads to low throughput. This throughput problem is exacerbated because the '745 invention requires that the substrate be moved to a clean reactor between successive deposition and etch operations for process stability. Further, with such conventional methods, gap-fill process margins are relatively narrow, with problems such as clipping (excessive etching) or voids within the filled gaps (excessive deposition).
As another attempt to improve the gap fill capability of the conventional HDP CVD methods, fluorine or fluorine compounds are added during the deposition process to improve gap fill. The fluorine in the form of free radicals acts as an etchant that works against the growth of material in the area of the trench openings. In other words, the free radicals chemically etch deposited material in the trench opening area. Consequently, the trenches can be filled with dielectric films from the bottom up without premature closing up of the trenches, leaving no voids therein (desirable bottom-up fill). For these reasons, in the prior art, the fluorine-containing gas flow rate has typically greater than the silicon source gas flow rate, e.g., greater than 1, for forming a silicon-oxide based gap-fill layer. This greater gas flow rate results in good gap-fill results because more fluorine species work against the growth of material in the area of the trench openings.
However, conventional HDP-CVD methods using fluorine-containing gas tend to incorporate an undesirable number of fluorine atoms that significantly degrade the device characteristics. For example, use of too many fluorine atoms can cause undesirable out-diffusion of fluorine atoms into the silicon substrate, delamination of layers and/or damage to silicon active regions. Attempts to overcome this problem by using a barrier layer to prevent out-diffusion of the fluorine atoms understandably complicates the overall manufacturing process, decreases throughput, and increases manufacturing costs.
Accordingly, what is needed is an improved high-density plasma (HDP) chemical vapor deposition (CVD) methods that overcomes such problems of the prior art.